Chandima Gomes

Concepts of lightning return stroke models

Return stroke models have been reviewed according to the assumptions they make and their underlying concepts. We have investigated the roots of the modern concepts used to describe return stroke related phenomena and illustrated their developments and deviations from the original idea. We discuss the return stroke models in two categories in accordance with the direction of the charge transfer along the channel. We show that all the models that we have considered can be described by two general sets of mathematical equations. The validation and comparison of models in reproducing measured lightning parameters are discussed. We also show the different methods employed by return stroke models to estimate the return stroke speed and discuss possible improvements that could be introduced into return stroke models in the future.

Comparison of preliminary breakdown pulses observed in Sweden and in Sri Lanka

Abstract In this paper we have analysed breakdown pulse trains preceding the first return stroke in 47 negative cloud-to-ground (CG) lightning flashes observed in Sri Lanka (tropics) and that in 41 negative CG flashes recorded in Sweden (temperate regions). In the data obtained in Sri Lanka, breakdown pulses could be detected only in nine flashes. In other flashes either these pulses are absent or, if they are present, their amplitudes should be below the background noise level. From the nine flashes with breakdown pulse trains, we obtained the following results. The ratio between the maximum breakdown pulse amplitude and the return stroke amplitude (BP\RS ratio), as a percentage, is 16.5%. The time duration between the most active part of the pulse train and the return stroke (BP\RS separation), is 11.9 ms and the ratio between the noise amplitude and the return stroke amplitude (N\RS ratio), as a percentage, for all 47 flashes, is 5.0%. In the flashes observed in Sweden, breakdown pulses are detectable in all records. In this data, the BP\RS ratio, as a percentage, is 101.4%, the BP\RS separation is 13.8 ms and the N\RS ratio, as a percentage, is 3.7%. All the above values are arithmetic means. Most of the pulses of breakdown pulse trains observed in both countries are bipolar in nature with the initial polarity the same as that of the succeeding return stroke.

Evaluation of electromagnetic fields associated with inclined lightning channel using second order FDTD-hybrid methods

Evaluation of electromagnetic flelds caused by the lightning channel is an appealing topic in order to consider the indirect efiects of lightning on the power lines. A common assumption for the calculation of electromagnetic flelds at the observation point is a vertical lightning channel, but the fact is that in reality the lightning channel is seldom vertical on the ground surface. In this study, the electromagnetic flelds due to inclined lightning channel at various observation points with difierent angles and with respect to the image of lightning channel on the ground surface were explored. This study also proposes general equations that can estimate the electric flelds due to inclined lightning channel through the 2nd FDTD method. The proposed method supports the notion of vertical lightning channel while the channel angle with respect to z-axis is assumed to be zero. This method was validated through the data gathered from flve flelds: three at a close distance from inclined lightning channel and two at intermediate distances from vertical lightning channel. Similarly, due to inclined lightning channel, the efiects of geometrical and current parameters on the electromagnetic flelds are considered. This study substantiates difierent coupling models with FDTD structure directly at the time domain without a need for extra converters.

Numerical expressions in time domain for electromagnetic fields due to lightning channels

In this paper, by considering the factors that influence the charge height variation along the lightning channel and using Maxwells equations, general expressions in time domain for the evaluation of electromagnetic fields at an observation point are proposed. The lightning channel is assumed to be perpendicular to the ground surface and the ground conductivity is considered to be perfect. Our results showed that the proposed method has good agreement with the measured values, as we validated the outcome using fields measured at close and intermediate distances from the lightning channel. A set of general equations, which has the ability of evaluating any given channel base current function and current model in time domain is also derived.

Analytical Expressions for Electromagnetic Fields Associated with the Inclined Lightning Channels in the Time Domain

Abstract This article describes general analytical equations that are developed for the calculation of electromagnetic fields due to an inclined channel directly in the time domain. The input parameters of the model, angle of inclination of the lightning channel and the angle at the observation point can be varied through all possible values to produce the relevant electromagnetic fields. The proposed model is validated with eight electromagnetic waveforms measured at close range of the lightning channel. The results of the model show good agreement with the measured data. Furthermore, the proposed equations are compatible with different channel base current functions. The equations could easily be applied in the coupling models that calculate induced voltages in conductors in the presence of lightning-generated electromagnetic fields.

Lightning accidents in Uganda

This paper addresses the lightning safety environment in Uganda that requires urgent and serious attention of the international expert community. We present recent statistics of reported lightning incidents in Uganda which show that the number of injuries and deaths are overwhelmingly high during year 2011 thus the situation can be declared critical. During the five-year period from January 2007 to December 2011, there were 91 reported lightning accidents which accounted for a total number of 150 deaths and 584 cases of personal injuries to which the victims did not succumb. Out of 150 deaths 95 reported in 2011. The actual number in each case may be much higher as many deaths and injuries go unnoticed due to lack of communication between rural communities and government authorities or media sources. The detailed analysis of several incidents reveals that injuries and deaths are basically due to both lack of awareness and absence of proper protection and safety schemes. The decisive factors of the ill-effects of incidents are time of the day, month of the year, part of the country, location of the incident etc. Most of the accidents have occurred either in the afternoon or in the evening. A spread of incidents can be seen throughout the six month period from June to November with June as the month of highest reported deaths. The highest number of incidents has been reported in the Northern Province of the country. Interestingly, the highest number of incidents, deaths and injuries has occurred while the victims were inside permanent structures. After analyzing these incidents a stepwise procedure is proposed to curb such incidents in the future.

Evaluation of lightning return stroke current using measured electromagnetic fields

The lightning return stroke current is an important parameter for considering the efiect of lightning on power lines. In this study, a numerical method is proposed to evaluate the return stroke current based on measured electromagnetic flelds at an observation point in the time domain. The proposed method considers all fleld components and the full wave shape of the current without the use of a special current model as a basic assumption compared to previous methods. Furthermore, the proposed algorithm is validated using measured flelds obtained from a triggered lightning experiment. The results show a good agreement between the simulated fleld based on the evaluated currents from the proposed method and the corresponding measured fleld at a remote observation point. The proposed method can determine current wave shapes related to a greater number of lightning occurrences compared to the direct measurement of the current.

Lightning safety of animals

This paper addresses a concurrent multidisciplinary problem: animal safety against lightning hazards. In regions where lightning is prevalent, either seasonally or throughout the year, a considerable number of wild, captive and tame animals are injured due to lightning generated effects. The paper discusses all possible injury mechanisms, focusing mainly on animals with commercial value. A large number of cases from several countries have been analyzed. Economically and practically viable engineering solutions are proposed to address the issues related to the lightning threats discussed.

Long impulse currents associated with positive return strokes

Long distant electric fields (400-500 km), generated by 26 positive cloud-to-ground flashes, were analysed, These flashes consist of well detectable long impulse fields subsequent to the initial pe ...

Lightning safety scheme for sheltering structures in low-income societies and problematic environments

This paper discusses various practical difficulties of implementing internationally and nationally recognized lightning protection standards on small shelters. In aiming to minimize human injuries and deaths due to lightning related accidents for people who take shelter in unprotected structures during thunderstorms, this study proposes out-of-the-box solutions that may save many lives in societies that cannot afford packages of total solution. However, the proposed protection schemes can be applied even in developed countries or wealthy societies where strategic problems such as remoteness of the sites of concern and practical solutions beyond the peripherals of standardized recommendations are demanded. It should be noted that these are last resort solutions rather than replacement for recommendations in international standards.